This invention relates to methods and apparatuses for attaching strain gage devices.
Strain gages are often provided with thin backing plates or flanges, that can be welded to a base by resistance spot welding. In such welding, one electrode presses down on the strain gage backing plate while another electrode presses on the base to carry current through the plate and base to weld them together. However, this method, while satisfactory for many materials, cannot be used on flight status aircraft or any other structure formed of a material such as titanium or Rene 41, which experiences a drastic reduction in fatigue strength as a result of the welding operation.
One method for attaching the gage while minimizing reduction in fatigue strength of the base material, includes brazing wherein a quantity of brazing material such as a silver or gold-base alloy is utilized which melts at a moderate temperature. A foil of such material can be placed between the gage backing plate and the base. Then, a pair of resistance welding electrodes can be utilized which press respectively against the backing plate and base to hold them together while a current is applied, the current being somewhat less than that utilized in resistance welding. However, the equipment and technique are relatively cumbersome for field installations, and the base is subjected to more heating than is desirable. A field installation technique for installing strain gages, which utilized relatively compact and easily manipulated equipment, and which minimized heating of the base material to which the strain gage backing plate is applied, would be of considerable value.